The present invention generally relates to metrology systems and, more particularly, to a metrology system that enables determination of the relative orientation of one point relative to another with five degrees-of-freedom and to a method for remotely measuring five degrees-of-freedom for a point target.
In an effort to enhance the performance and functionality of modern large space structures, such as satellites and other spacecraft, one class of structural designs increasingly features extended structures. Such structures may include large planar or boom-type structures that may have alignment-critical elements. In the case of widely spaced but structurally connected alignment-critical elements, however, it becomes increasingly difficult to maintain the alignment of the elements using rigid structural designs alone. Other means of compensating the deformation of the structure must be employed. However, in order to compensate for structural deformation, it must be measured. In many cases detailed deformation information is not needed. Instead the largest deformations often arise from lowest-order modes; the majority of the effects on the system may be deduced from measuring these modes, or at a minimum measurement of the relative orientation of critical components.
One example is the case of large fixed antenna reflectors and antenna feeds that lie at opposite ends of a boom connecting them. While increasing the size of the reflector and the spacing from the feed can increase antenna performance from an electromagnetic point of view, the required precision and stability of the structure becomes harder to maintain. For instance, the relative positioning between the reflector and feed components becomes increasingly difficult to maintain and/or control as the size of the space structure, and in particular the length of the boom and the size of the fixed antenna reflector, increases.
Additional concerns arise because the structure mass is limited by the launch vehicle's lift capacity. The desire is to maximize the utility of the mass that is launched; hence in the case of large fixed antenna structures, a fixed launch mass implies that increases in system performance come as the efficiency with which mass is utilized in the space structure is increased. Consequently, the mass density of the space structure is likely to drop and the dynamic stability of the resulting structure will decrease. As a result, more complex deformation modes will be introduced and will need to be compensated in order to maintain the desired performance gains. Any such distortion compensation system will require a high precision metrology system.
For many of the near-term projected distortion compensation problems, some of the existing metrology systems currently deployed on space structures may, with sufficient engineering, provide the needed measurement precision and accuracy. However, these systems have multiple implementation issues that limit their use. For instance, currently existing metrology systems may be relatively expensive and heavy, requiring extensive cabling between the metrology system and the remote point to be measured, scanning mirrors and motors. Furthermore, currently existing metrology systems may be complex to execute, for instance, requiring digital processing of the received data. However, while these systems may be able to meet near-term needs, the measurement precision and accuracy of currently existing metrology systems may not scale as well as will be needed to control future large space structures.
Prior art, for example, U.S. Pat. No. 6,293,027 B1 issued to Elliott et al., discloses a technique for measuring distortion of spacecraft structures in applications that require extremely precise dimensional relationships. The disclosed prior art system includes a first set of targets affixed to a spacecraft structure, a first target scanning module affixed to a reference point on a frame of reference that includes means for measuring a range and an angular position for each of the first set of targets, means for computing the orientation of the spacecraft structure relative to the frame of reference from the measured ranges and angular positions of the first set of targets, a second set of targets affixed to the spacecraft structure, a second scanning module affixed to a reference point on the spacecraft structure that includes means for measuring a range and angular position for each of the second set of targets, and means for computing shape distortion measurements pertaining to the spacecraft structure from the measured ranges and angular positions of the second set of targets. From this set of measurements, and a model for the geometrical and structural arrangement, measurement of two- or three-dimensional deformation of the structure can be made. The prior art system disclosed by Elliott et al. is relatively complex, including scanning mirrors that are rotated and therefore, may be prone to mechanical failure during operation in space. Furthermore, the geometrical arrangement of determining angular position of targets does not scale well as the size of the structure increases and, in general, such a scheme will have geometrically decreased accuracy at longer distances.
Prior art further includes, for example, a method for measuring inter-story drift in smart buildings using laser crosshair projection disclosed by Bennett and Batroney in Optical engineering, Vol. 36 No. 7, July 1997, pages 1889–1892. Bennett and Batroney utilize position sensitive photo detectors to determine the position of a laser generated crosshair. By applying this prior art method, the relative orientation of one point relative to another with 3 degrees-of-freedom can be determined. The 3 degrees-of-freedom are lateral translation in the x and y dimension, and roll rotation of one point relative to another. This system has the advantage that, as distances are increased, the measurement precision theoretically does not decrease; rather, only the range of displacement that can be measured on the detector decreases as the size of the laser-generated cross hair becomes increasingly diffuse at longer distances from the laser source. However, Bennett and Batroney neither consider, disclose, nor suggest measurement of the other degrees of freedom, such as pitch and yaw rotations of the target. Additionally, the method of Bennett and Bartroney may be unable to differentiate between lateral translations and rotations.
As can be seen, there is a need for a high-precision metrology system that can measure a plurality of degrees of freedom of low-order deformation modes, in order to facilitate active compensation of large spacecraft structures. Furthermore, there is a need for a metrology system that enables measurements of deformations of large space structures with alignment-critical elements deployed across increasingly high aspect-ratio mass efficient structures, for example, large spacecraft structures having a large fixed reflector connected with a boom and an antenna feed. Also, there is a need for a metrology system with measurement precision that scales well as structural size increases, and that provides a significant reduction in relative complexity and an improvement in robustness, as well as a higher performance with lower-cost components. Moreover, there is a need to provide a method for determining the relative orientation of one point relative to another with at least 5 degrees-of-freedom including lateral translation in x, y dimension as well as roll, pitch, and yaw rotation. In some circumstances, measurement of a sixth degree of freedom, distance z between two points, may also be desirable. Furthermore, there is a need to provide a method that enables differentiation between lateral translations and rotations.